1. Field of the Invention
This invention relates generally to solid state image sensors comprising a charge transfer device, and more particularly to such solid state image sensors of the interline transfer type which are improved in structure so that the charge transfer efficiency in the vertical charge transfer portion is increased.
2. Description of the Prior Art
Solid state image sensors comprising a charge transfer device such as a charge coupled device (hereinafter referred to as a CCD) are classified broadly rnto the frame transfer type and the interline transfer type, and each of these is utilized in different ways so as to make good use of its advantages and features. The solid state image sensor of the interline transfer type using a CCD (hereinafter referred to as an interline transfer CCD image sensor) comprises a sensing and vertical transfer portion including a plurality of photo-sensing areas provided to make horizontal rows and vertical rows, vertical charge transfer portions formed with a group of CCD and provided along each of the vertical rows of the photo-sensing areas and transfer gate areas provided between each of the photo-sensing areas and the corresponding one of the vertical charge transfer portions. A horizontal charge transfer portion is formed for a different group of the CCDs and is coupled to the photo-sensing and vertical transfer portion and an output portion is coupled to the horizontal charge transfer portion and is provided with a signal output terminal. On the vertical charge transfer portions and the horizontal charge transfer portion, suitable transfer electrodes are provided for performing the transfer of the charges in the respective portions and a vertical transfer clock signal and a horizontal transfer clock signal are supplied thereto. The photo-sensing area produces a signal charge in response to the light received and stores the produced signal charges therein, and the signal charges are transferred to the vertical charge transfer portion through the transfer gate area and further transferred to the horizontal charge transfer portion during each period corresponding to a horizontal blanking period by the charge transfer operation of the vertical charge transfer portion. The signal charges transferred to the horizontal charge transfer portion during each one of the periods corresponding to the horizontal blanking periods is transferred to the output portion during a period corresponding to a horizontal video period and an image pickup signal output is obtained at the signal output terminal.
In the previously proposed interline transfer CCD image sensor, an undesirable potential difference is unavoidably induced between an area in the vertical charge transfer portion which is contiguous to the transfer gate area and the other area in the vertical charge transfer portion. This potential difference in the vertical charge transfer portion reduces the charge transfer efficiency in the vertical charge transfer portion and such a reduction of the charge transfer efficiency prevents miniaturization of the vertical charge transfer portions for increasing the sensitivity of the sensor and for miniaturizing the entire sensor and establishes a limitation on the design of the sensor. This is a disadvantage of previously proposed interline transfer CCD image sensors.